1. Field of the Invention
The present invention relates to a hydrogen sensor.
2. Description of the Related Art
To detect hydrogen with a high sensitivity, sensors of various systems such as optical sensors, catalytic combustion sensors, semiconductor sensors, electromotive force type sensors, current detection (battery) sensors, mechanical type sensors utilizing pressure variation that employ hydrogen adsorption or hydrogen occlusion characteristics, and MOS capacitor sensors have been developed and used in different applications in which the characteristics of each system can be exhibited sufficiently. For example, to detect gas leakage, semiconductor sensors made of a material such as TiO2, SnO2 or the like generally are used. For gas leakage of industrial or combustion equipment or temperature control, catalytic combustion sensors generally are used.
In recent years, hydrogen sensors made of, for example, SrCeYO3, which is a proton conductor, have been developed as a sensor that can detect hydrogen in a solution in which Al or the like is molten at a high temperature.
Furthermore, a sensor using a calcium-zirconium oxide, which is an oxide ion conductor (oxide ions are represented by MO−, MxOyα− or the like, in addition to O2−) for a solid electrolyte, is known as a hydrocarbon sensor that detects combustible gas, which is the same as a hydrogen sensor. This sensor is characterized by excellent hydrogen selectivity, although the configuration is simple. For example, electromotive force type sensors employing Pd-Au electrodes (see the Meeting Abstract of the Spring Meeting of the Electrochemical Society '95, University of Nagoya) and limiting a current detection type sensors employing porous alumina as a diffusion controlled layer (see the proceedings of the autumn conference of the Chemical Sensor Society '96, TOYOTA Central R&D Lab., Inc.) are well known.
Thus, a proton conductor such as a SrCeO3-based oxide and a CaZrO3-based oxide, or an oxide ion conductor such as zirconia and ceria is used as an ion conductor that is used for a solid electrolyte, and various hydrogen sensors with high sensitivity and high hydrogen selectivity have been proposed.
However, inexpensive hydrogen sensors that have excellent hydrogen selectivity and can measure hydrogen even in a high concentration region have not been developed. For example, optical sensors are suitable for precise analysis, but they are expensive.
Although catalytic combustion sensors and semiconductor sensors are comparatively inexpensive and have high reliability during measurement for a long time, they have poor hydrogen selectivity because they react with a reducing gas such as CO that is not an immediate subject for measurement, so that they are not suitable for detection of a high concentration of % order. Furthermore, in semiconductor sensors, the hydrogen selectivity is increased by mixing a semiconductor material used with a catalytically active material, but they are not suitable for measurements for high concentration regions.
In electromotive force type sensors and a current detection type sensors employing a calcium-zirconium oxide, which is an oxide ion conductor, for a solid electrolyte, the detection precision of hydrogen concentrations depends on the characteristics of the ion conductor. In addition, the proton conductivity of the ion conductor is small (about 5×10−4S/cm at 600° C.), and the detection precision of hydrogen concentrations is low. Therefore, for electromotive force type sensors, it is necessary to set the temperature of a sensor to a high temperature of 700° C. or more during detection of hydrogen concentrations, and for a current detection type sensors, it is necessary to reduce the thickness of the solid electrolyte. Mechanical type sensors utilizing pressure variation and MOS capacitor sensors have poor responsiveness to detection, and low practical values.
At present, there is a demand for hydrogen sensors for detecting hydrogen in living environments, selectively detecting hydrogen leaked from a fuel battery, and detecting the concentration of hydrogen contained in the air at room temperature or hydrogen in an airtight room. In particular, inexpensive hydrogen sensors that have excellent hydrogen selectivity and good responsiveness and can detect hydrogen in a high concentration region in a simple manner are desired as a sensor for controlling a fuel battery and equipment utilizing hydrogen, but as described above, hydrogen sensors that satisfy these requirements sufficiently have not been obtained yet.